Electrically conductive compositions for forming electroconductive connections between conductive members, i.e., termination pads on a chip and pads associated with circuits printed on a circuit board, are well known in the art. Indeed, conventional solder compositions which comprise eutectic alloys, such as lead-tin alloys, have long been known in the art. Such materials, however, require the inclusion of a flux composition to remove oxides resident on the surface of the solder metals. These flux materials which are acidic in nature, are corrosive to the highly valuable electrical components that are so electrically connected, and are usually removed by the use of solvents. However, traditional flux removal solvents, such as CFCs, are environmentally objectionable and their use complicates the manufacturing procedures wherein flux materials are utilized. Thus, flux materials find less and less application in electrical assemblies requiring the connection of expensive electrical components used in modern electrical and electronic devices such as computers and the like.
New electrically conductive pastes have been developed in view of this known disadvantage of eutectic alloys. Typically, these pastes comprise electrically conductive metal powders and a thermosetting resin material. Such materials are applied to the electrical components then heated to form an electrical connection. During that heating the thermosetting resin cures. Although it is known to utilize a nonoxidizing metal as the electrically conductive powder in these new pastes, thus avoiding the necessity of utilizing a flux, such materials are characterized by the disadvantageous property of not being remeltable. Those skilled in the art are aware that upon curing of a thermosetting resin, a crosslinked structure is formed. Polymers having a crosslinked structure cannot be remelted or dissolved. Thus, such electrically connected components, which can be quite expensive, cannot be easily disconnected and reconnected, when necessary, by merely heating or redissolving the conductive paste to disconnect the components and thereafter reconnecting them using the previously used or fresh identical paste.
A recent development, which provides a significant advance in the art, is described in U.S. Pat. No. 5,062,896 which is incorporated herein by reference. The '896 patent is directed to a composite poly-mersolder paste which included a eutectic metal alloy powder filler, said alloy having a melting point below 200.degree. C., present in a concentration of about 85% to 93% by weight, based on the total weight of the paste.
The paste composition of the '896 patent included, as a second component, a thermoplastic polymer. It is the presence of that thermoplastic polymer which represents the advance in the art provided by this patent. The thermoplastic polymer of the paste of the '896 patent permits remelting of the paste. As those skilled in the art are aware, a thermoplastic polymer does not cure into a three-dimensional network. Rather, the thermoplastic polymer, which in the '896 patent is preferably a poly(imide siloxane), because it essentially does not crosslink, can be remelted or redissolved.
A third component included in the paste of the '896 patent is a volatile organic solvent having a boiling point above the melting point of the alloy powder but below the maximum reflow temperature of the composite paste composition.
Finally, a fourth and last essential component of the '896 composite paste is a transient flux material which is an aliphatic monocarboxylic acid having a boiling point between about 140.degree. C. and 200.degree. C. It is noted, however, that an aromatic monocarboxylic acid, such as 2-methoxybenzoic acid, may also be utilized. The flux material is generally present in an amount of between about 0.5% and 1.5% by weight.
Although the paste composition of the '896 patent, which may include optional components such as surfactants and the like, represents a significant advance in the art, solving as it does the problem associated with remelting of metal powder-polymer composites, does not overcome the other major problem associated with the materials of the prior art. That is, the problem associated with acidic fluids, which evolve from the fourth essential transient flux material and have a detrimental corrosive effect upon the expensive electronic components which are electrically connected by the paste, is unaddressed.
A flux component must be included in the paste of the '896 patent because of the inclusion therein of oxidizable metals, as made clear by the use therein of metals having a melting point of less than 200.degree. C. As such, the advance in the art provided by the paste of the '1896 patent does not address all the problems associated with the electrically conductive interconnecting materials of the prior art.